1. Field of the Invention
The present invention relates to a data recording apparatus, a data recording method, a data recording and reproducing apparatus, a data recording and reproducing method, a data reproducing apparatus, a data reproducing method, a data record medium, a digital data reproducing apparatus, a digital data reproducing method, a synchronization detecting apparatus, and a synchronization detecting method that are used for recording and/or reproducing a digital video signal and a digital audio signal.
2. Description of the Related Art
A data recording and reproducing apparatus that records a digital video signal and a digital audio signal to a record medium and that reproduces a digital video signal and a digital audio signal therefrom is known. A typical example of such an apparatus is a digital VTR (Video Tape Recorder). In a record processing portion of a digital video signal recording apparatus, digital video data and digital audio data are placed packets with a fixed length. ID information is added to each packet. The packetized data is encoded with an error correction code. A synchronous pattern and ID information are added to packetized data, an error correction code parity, and so forth so as to form a sync block. A plurality of sync blocks are grouped as a sector corresponding to each data type. Each sector as serial data is recorded on a magnetic tape by a rotating head. The length of each sync block in the same sector is the same. The sync blocks are successively assigned unique ID numbers. The ID information has the same value. A product code is used as an error correction code. In other words, a two-dimensional array of data symbols is encoded with an outer code in the vertical direction and an inner code in the horizontal direction. Thus, each symbol is dually encoded. One minimum data encoding/decoding unit of the product code is referred to as ECC block.
On the reproducing side, the start position of each sync block is detected with a synchronous signal. Packets in each sync block are rearranged corresponding to ID numbers and ID information. Since a unique synchronous pattern is added at the start position of each sync block, using the bit sequence of the synchronous pattern, the pattern occurrence interval, successive ID numbers in the same sector, and the same ID information, the phase of a synchronous block can be detected. In other words, when the conditions that the bit sequence of a synchronous pattern matches a fixed pattern, that the same pattern is detected at a position delayed by the block length, and that the block ID is proper are satisfied, the phase of the synchronous block is detected. In the format of such a conventional digital VTR, to easily perform the synchronization detecting process, the length of each synchronous block is fixed (to one type) regardless of the data type.
To record and reproduce video data, a compression encoding process is performed. When video data corresponding to MPEG (Moving Picture Experts Group) standard is compression-compressed, coefficient data generated by DCT (Discrete Cosine Transform) process is encoded with a variable length code. When the amount of data that is recorded per track or every a predetermined number of tracks is fixed, the data amount of the variable length code that is generated in a predetermined time period is limited to a predetermined value. Variable length code encoded data (namely, variable length data) is packed in data areas of a plurality of sync blocks corresponding to a predetermined time period.
The data amount of a digital audio signal is not so large in comparison with that of a digital video signal. To prevent the audio quality from deteriorating in the compressing process and to prevent a complicated process because the data access unit of an MPEG audio signal does not match a video frame and a video signal and an audio signal are switched, non-compressed audio data (linear PCM) is recorded and/or reproduced.
There are as many as 18 types of digital television broadcasting formats in the United States. In such an environment, a digital VTR that can record and reproduce video data in a plurality of formats is desired. When the length of each sync block is fixed to one type regardless of data types as with the conventional digital VTR, although synchronization is easily detected, it is difficult to record data in various formats. Next, this point will be described.
Next, an example of the conventional digital VTR will be described. The VTR records video data and audio data on a tape in a tape format as shown in FIG. 1A. As shown in FIG. 1A, data of six tracks is recorded per frame. One segment is composed of two tracks with different azimuths. In other words, six tracks are composed of three segments. A pair of tracks that compose one segment are assigned track numbers [0] and [1] corresponding to the azimuths. Video sectors are formed on both edges of each track. Video data is recorded on the video sectors. An audio sector is formed between the two video sectors. Audio data is recorded on the audio sector.
In the track format shown in FIG. 1A, audio data of four channels can be handled. Referring to FIG. 1A, A1 to A4 represent sectors of channels 1 to 4 of audio data, respectively. The video data is shuffled (interleaved) and recorded on sectors on the upper side and the lower side. A system area (sys) is formed at a predetermined position of each video sector on the lower side. In FIG. 1A, SAT1(Tr) and SAT2 (Tm) are areas in which a servo lock signal is recorded. In addition, gaps (Vg1, Sg1, Ag, Sg2, Sg3, and Vg2) with predetermined sizes are formed between individual record areas.
As shown in FIG. 1B, data recorded on the tape is composed of a plurality of blocks that are equally divided (these blocks are referred to as sync blocks). FIG. 1C shows an outlined structure of one sync block. One sync block is composed of an ID (that identifies the current sync block), a DID (that represents the contents of data that follows), a data packet, and an error correction inner code parity. Data is recorded and reproduced as sync blocks (the minimum data recording/reproducing unit is one sync block). For example, a video sector is composed of many sync blocks that are arranged.
One sync block is composed of a synchronous signal, an ID, a data packet, and an inner code parity. Now, one sync block is denoted by                sync block: sync pattern+sync id+data packet        +inner parity.        Design condition: The length of one data packet of video data is the same as the length of one data packet of audio data.        
Next, as an example of the recording process of video data, the following video data and conditions are considered.                Video data (4:2:2)        Design conditions: Data compression ratio=2 or more (the data amount after data compressing process is ½ or less of the data amount before data compressing process).        10 DCT blocks are packed to two sync blocks.        6 tracks per field.        [525 lines/60 fields] format video signal        
Amount of video data per field:                512×720×(8+4+4) bits/8/2=368640 bytes        
Number of DCT blocks per field:                512×720/8/8=5760        
10 blocks/2 syncs/→1152 sync blocksLength of data packet>368640×(1/2)/1152=160   (1)                [625 lines/60 fields] format video signal        
Amount of video data per field:                608×720×(8+4+4) bits/8/2=437760 bytes        
Number of DCT blocks per field:                608×720/8/8=6840        
10 blocks/2 syncs/→1368 sync blocksLength of data packet>437760×(1/2)/1368=160   (2)
An example of the recording process for audio data is as follows:                Audio data (24 bits, 48 kHz sampled)        Design condition: Non-compression        AUX data: 6 bytes per field        
Number of samples per field in [525/60] format:                48 k/59.94 Hz×24 bits/8=2402.4 bytes                    (5 field sequence)                        AUX data of 12 bytes→2415 bytes (total data amount)        Number of samples per field in [625/50] format:                    48 k/50 Hz×24 bits/8=2880 bytes                        AUX data of 12 bytes→2892 bytes (total data amount)        
To determine the optimum sync block length of audio data, the products of data packet lengths (162 and 163) and the numbers of sync blocks are obtained as follows.
15161718161:2415257627372898162:2430259227542916
Now, it is defined that the video compression rate is the ratio of the data amount of video data that has been compressed and the data amount of original video data. The data packet length is selected so that the video compression rate becomes 2 or more. The data packet length of which the excessive record area of audio data in both the [525] format and [625] format is 161. However, since each audio sample is composed of 24 bites (3 bytes), the data packet length should be a multiple of 3. Thus, the data packet length should be 162. Consequently, in the digital VTR format, the data amounts are defined as follows.                [525/60] format video data: 162×1152=186624 bytes                    audio data: 162×15=2430 bytes                        [625/50] format video data: 162×1368=221616 bytes                    audio data: 162×18=2916 bytes                        
Error correction outer code parity data is added to each of video data and audio data. The number of outer code parities added to video data is 10% thereof. The number of outer code parities added to audio data is 100% thereof. (In other words, the number of audio symbols is the same as the number of parities.) Since the circuit scale largely depends on the number of parities, the maximum number of parities is limited to 14. In addition, the number of tracks per field is 6. Thus, the sum of the number of data blocks and the number of outer code parities should be divided by 6. In the case of video data, two ECC blocks are formed on one track.
[525/60] format video data                1152=(96×2)×6→Number of outer code parities                    =10                        2 ECC blocks per track        Number of data blocks per track+number of outer code                    parities=(96+10)×2=212                        
[625/50] format video data                1368=(114×2)×6→Number of outer code parities                    =12                        2 ECC blocks per track        Number of data blocks per track+number of outer code                    parities=(114+10)×2=248                        
In the case of audio data, one ECC block is formed in one field.
[525/60] format audio data
15=(5×3)→Number of outer code parities=5
3 ECC blocks per field                Number of data blocks per track+number of outer code                    parities=(15+15)/6=5.                        Number of bytes in unnecessary record area per CH                    =21 bytes/field                        
[625/50] format audio data                18=(9×2)→Number of outer code parities=9        2 ECC blocks per field        Number of data blocks per track+number of outer code                    parities=(18+18)/6=6                        Number of bytes in unnecessary record area per CH                    =30 bytes/field                        
An ID (2 bytes), a block synchronous signal (sync pattern) (2 bytes), and an inner code parity (14 bytes) are added to each data packet and thereby a sync block (180 bytes each) is formed as record data. Thus, video data and audio data are recorded as sync blocks on a tape. The decoder detects the beginning of each sync block with the synchronous signal, corrects an error thereof with an inner code, separates each sync block into a video sync block or an audio sync block with a video/audio data identification flag recorded in the ID, corrects an error of each of a video sync block and an audio sync block with an outer code, and decodes the video sync block and audio sync block to video data and audio data.
Each sync block of video data and each sync block of audio data are structured so that the length of the former is the same as that of the latter. Thus, the beginning of each sync block can be easily detected. FIGS. 2A and 2B show ECC block structures of a conventional digital VTR. FIG. 2C shows the structure of one sync block. FIG. 2A shows the structure of a video ECC block. FIG. 2B shows the structure of an audio ECC block. As shown in FIG. 2C, the length of each video sync block is 180 bytes. The length of each audio sync block is 180 bytes. Thus, the length of each video sync block is the same as the length of each audio sync block. In the [625/50] format and [525/60] format, one video ECC block (FIG. 2A) is structured in such conditions that the number of blocks per frame is 12, that the number of heads is 4, and that the number of tracks per frame is 6. In the [625/50] format, one audio ECC block (FIG. 2B) is structured in such conditions that the number of blocks per frame is 1, that the number of heads is 4, and that the number of tracks per frame is 6. In the [525/60] format, one audio ECC block is structured in such conditions that the number of blocks per frame is 1, that the number of heads is 4, and that the number of tracks per frame is 6.
FIGS. 3 and 4 show the relation between an audio ECC block and audio samples. FIG. 3 shows the arrangement of samples in the case that the field frequency is 50 Hz. FIG. 4 shows the arrangement of samples in the case that the field frequency is 59.94 Hz. In FIGS. 3 and 4, audio sample numbers starts from the beginning of the current field. AUX is system data that represents the contents of audio data. The arrangement of samples and the structure of one ECC block in the [525/60] format (FIG. 3) are different from those in the [625/50] format (FIG. 4). Thus, the audio encoder and the audio decoder each require a circuit that changes a process corresponding to a selected mode.
Next, a multi-rate format will be considered. In the format that the video rate of the conventional VTR format is decreased by 3, in formulas (1) and (2), when ½ is substituted with ⅓, the length of each data packet becomes 107. On the other hand, when the length of each audio data packet is the same as the length of each video data packet, since the length of each video data packet should be a multiple of the number of audio samples (3 bytes), the length of each video data packet becomes 108.
The data amount of audio data per field is 2415 bytes in the [525/60] format and 2892 bytes in the [625/50] format. Thus, in the [525/60] format, the data amount of audio data per field becomes 108×3=2484 bytes. In the [625/50] format, the data amount of audio data per field becomes 108×27=2916 bytes.
Combinations of the data packet length (108 bytes) and the number of sync blocks (the product thereof represents the total data amount) are for example:
22232425262728108:2376248425922700280829163024
Next, the structure of each ECC block will be considered. In the case of video data, two ECC blocks are formed per track.
[525/60] format video data                1152=(96×3)×4→Number of outer code parities                    =10                        3 ECC blocks per track        Number of data blocks per track+number of outer code                    parities=(96+10)×3=318                        
[625/50] format video data                1368=(114×3)×4→Number of outer code parities                    =12                        3 ECC blocks per track        Number of data blocks per track+number of outer code                    parities=(114+12)×3=378                        
In the case of audio data, it is assumed that one ECC block is formed in one field. In this case, the number of tracks per field is 4.
[525/60] format audio data                23=23×1→Number of outer code parities=23        1 ECC block per field        Number of data blocks per track+Number of outer                    code parities=(23+23)/4=11.5                        
[625/50] format audio data                27=(9×3)→Number of outer code parities=9        3 ECC blocks per field        Number of data blocks per track+Number of outer                    code parities=(27+27)/4=13.5                        
In this case, in the NTSC system, the number of outer code parities is too large. Moreover, in both the cases, the number of blocks per track is not an integer. In other words, an ECC block cannot be formed. Thus, in the [525/60] format, 108×24=2592 bytes is selected; and in the [625/50] format, 108×28=3024 bytes is selected.
[525/60] format audio data                24=(8×3)→Number of outer code parities=8        3 ECC blocks per field        Number of data blocks per track+Number of outer code                    parities=(24+24)/4=12                        Number of bytes in unnecessary record area per CH=                    183 bytes/field                        
[625/50] format audio data                28=(7×4)→Number of outer code parities=7        4 ECC blocks per field        Number of data blocks per track+Number of outer code                    parities=(28+28)/4=14                        Number of bytes in unnecessary record area per CH                    136 bytes/field                        
In this example, in the [525/60] format, a loss record area of 138 bytes×4 ch per field (equivalent to 0.35 M bps) takes place. Thus, the record efficiency deteriorates. The loss area is proportional to the number of audio channels.
FIG. 5A shows the structure of a video ECC block whose video rate is changed from ½ to ⅓. FIG. 5B shows the structure of an audio ECC block whose audio rate is changed from ½ to ⅓. FIG. 5C shows the structure of a sync block in the case that the length of one video sync block is the same as the length of one audio sync block. In the [625/50] format and [525/60] format, one video ECC block (see FIG. 5A) is structured in such conditions that the number of blocks per field is 18, that the number of heads is 4, and that the number of tracks per field is 4. In the [625/50] format, one audio ECC block (see FIG. 5B) is structured in such conditions that the number of blocks per field is 4, that the number of heads is 4, and that the number of tracks per field is 4. In the [525/60] format, one audio ECC block is structured in such conditions that the number of blocks per field is 3, that the number of heads is 4, and that the number of tracks per field is 4.
FIG. 6 shows the relation between an audio ECC block and audio samples. FIG. 6 shows the arrangement of samples with a field frequency of 50 Hz. The arrangement of samples shown in FIG. 6 is largely different from that of original samples shown in FIGS. 3 and 4. The multi-rate type VTR should also record and reproduce the original format data, it should process data of all different arrangements. Thus, the multi-rate type VTR requires signal processing circuits corresponding to all formats of various video data rates and various frame frequencies. Thus, the circuit scale of the multi-rate type VTR becomes large (because of a rise of the IC cost).
Actually, as shown in FIG. 7, 14 formats are considered as combinations of video data rates (25 M bps to 600 M bps), video scan modes (interlace and progressive), and frame frequencies (59.94 Hz, 50 Hz, 29.97 Hz, 25 Hz, and 23.976 Hz). In FIG. 7, an NTSC picture frame is composed of 720×480 and a PAL picture frame is composed of 720×576. The interlace mode and progressive mode as video scan modes are denoted by i and p, respectively.
It is necessary to define the lengths of sync blocks for all the formats shown in FIG. 7. The length of each sync block closely relates to the frame frequency, the data amount of video data, the data amount of audio data, and so forth. Thus, when the length of each video sync block is the same as the length of each audio sync block, it is very difficult to select the length (data packet length) that is optimum and common in all the formats. In addition, since the structure of audio data is largely affected by the video rate, circuits corresponding to all video rates should be disposed. If the processes performed by the multi-rate type encoder and decoder are different in the individual formats, the circuit scale becomes huge. Thus, the IC cost rises.
In the conventional digital VTR, one packet of variable length data is placed in one sync block. Thus, in the multi-rate format, the packet rate is proportional to the bit rate. However, since a sync pattern, an ID, and so forth that are added to each sync block have fixed lengths, the size of these data becomes large in the entire size of one sync block. In other words, the redundance of data becomes high.
In addition, since the synchronization detecting circuit of the conventional VTR reproducing system has only one synchronous pattern detecting portion, if input data has a plurality of types of sync blocks with different lengths, the circuit cannot correctly detect a sync pattern.